Drift-free avalanche breakdown diode

ABSTRACT

An avalanche breakdown diode includes a p-doped trough in which a highly p-doped region is introduced. In addition to the trough, an n-doped region is introduced, which is underlaid by a p-doped layer. The trough and the p-doped layer define a precisely established interspace. The arrangement is introduced into a p-type substrate. An insulating layer and thereon, in turn, a conductive layer are applied over the region between the trough and the p-doped layer. The conductive layer and the n-doped region are connected to a positive voltage and the highly p-doped region is connected to a negative voltage. A drift of the breakdown voltage is thereby prevented. In addition, the resistance during the breakdown is small due to the defined interspace between the trough and the layer.

FIELD OF THE INVENTION

The present invention relates to a diode, in particular a drift-freeavalanche breakdown diode. An avalanche breakdown diode constructed on ap-type substrate is already known, the anode being formed by a highlyp-doped trough, into which a very highly p-doped region is introduced,and the highly p-doped region being connected to a voltage supply. Thecathode is formed by a highly doped n-type region, which is underlaid bya highly p-doped region and is introduced into the p-type substrate. Thehighly n-doped region is connected to a positive voltage.

SUMMARY OF THE INVENTION

The arrangement according to the present invention has the advantagethat the drift of the breakdown voltage is minimized. This is achievedby preventing the breakdown from taking place in the vicinity of thesurface of the p-type substrate. The process for producing an avalanchebreakdown diode according to the present invention also has theadvantage that the p-doped layer and the p-doped trough areautomatically adjusted with respect to one another. In this case, theinterspace between the highly doped p-type trough and the highly p-dopedlayer is established exactly by the production process. This leads to asmall resistance during the breakdown of the avalanche breakdown diode.Consequently, this avalanche breakdown diode is suitable as an accuratevoltage reference. It is particularly advantageous to produce theinsulating according to the present invention from silicon oxide. Anadvantageous selection of the material for the conductive layeraccording to the present invention is the use of polysilicon.

The avalanche breakdown diode according to the present invention alsocan be produced advantageously in the form of an inverse conductivitystructure, that is to say arranging p-type material instead of n-typematerial. Consequently, the use of the avalanche breakdown diode isindependent of the semiconductor processes used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an integrated avalanche breakdown diode according to thepresent invention.

FIG. 2 shows doping profiles according to the present invention.

FIG. 3 shows a first ion implantation according to the presentinvention.

FIG. 4 shows two diffused regions according to the present invention.

FIG. 5 shows a second ion implantation according to the presentinvention.

FIG. 6 shows a further diffused region according to the presentinvention.

FIG. 7 shows a third implantation according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a p-type substrate 1, into which a highly p-doped trough 3is introduced. A very highly p-doped first region 4 is arranged in turnin the trough 3. In addition to the trough 3, a highly n-doped secondregion 6 is introduced. The second region 6 is underlaid by a highlyp-doped layer 2. The layer 2 and the trough 3 are separated by aninterspace 5 which has a precisely defined width. The interspace 5 isfilled by the p-type substrate 1. An insulating layer 7 is arranged overthe interspace 5. A conductive layer 8 is applied to the insulatinglayer 7. The conductive layer 8 and the second region 6 are connected toa second potential, a positive voltage. The first region 4 is connectedto a first potential, a negative voltage.

Arranging n-doped material instead of p-doped material, that is to sayconstructing the avalanche breakdown diode with the aid of an inverseconductivity structure, is self-evident to a person skilled in the art.

In this exemplary embodiment, 1.5×10²⁰ ions per cm³ were selected forthe doping of the highly n-doped second region 6. In this case, thedoping depth is 0.4×10⁻⁶ m. A doping of 2×10¹⁷ ions per cm³ was selectedfor the highly p-doped trough 3 and the highly p-doped layer 2, thedoping depth being 1.6×10⁻⁶. The very highly p-doped first region 4 hasa doping of 4×10¹⁹ ions per cm³, the doping depth being 0.6×10⁻⁶ m.

The arrangement according to FIG. 1 functions as follows: when abreakdown voltage is applied to the avalanche breakdown diode, theconductive layer 8 is simultaneously connected to the positive voltage,with the result that a high potential does not build up under theconductive layer 8 and therefore a breakdown is prevented in thisregion. In this arrangement, the breakdown takes place between thesecond region 6 and the layer 2, in the region where the second region 6is arranged horizontally on the layer 2. In this way, it is achievedthat the breakdown voltage remains constant, irrespective of the currentflowing and the voltage applied. Since the breakdown is not produced inthe vicinity of a surface layer, oxide charges, which produce long-termimpairment of the mode of operation of the avalanche breakdown diode,are prevented from forming.

FIG. 2 shows the characteristic of the doping profiles corresponding tothe lines of intersection A₁ -A₂ and B₁ -B₂ drawn in FIG. 1, the dopingin ions/cm³ being plotted against the line of intersection. On accountof the production process used, the doping profile in the verticaldirection A₁ -A₂ is very similar to the doping profile in the horizontaldirection B₁ -B₂. This produces a similar breakdown behavior in thevertical and horizontal directions. Due to the use of the conductivelayer 8 and of the corresponding voltage supply of the layer 8, anelectric field is produced in the interspace 5, which electric fieldprevents an avalanche breakdown in the horizontal direction B₁ -B₂, withthe result that the avalanche breakdown diode breaks down in thevertical direction A₁ -A₂.

The avalanche breakdown diode is produced according to a self-adjustingprocess. The process is explained using FIGS. 3 to 7. For theproduction, a p-type substrate 1, for example, is used, to which aninsulating layer 7 and a polysilicon layer 8 having a defined width areapplied. Positively charged ions are implanted into the p-type substrate1, as is illustrated in FIG. 3. On account of the width of theinsulating layer 7 and of the polysilicon layer 8, two positively dopedsurface layers 10 having a defined separation are obtained. Diffusionthen takes place in such a way that a positively doped trough 3 and apositive layer 2 are obtained, which have an established interspace 5having a defined width under the insulating layer 7, as is illustratedin FIG. 4. Subsequently, the positively doped layer 2 is covered with aphotoresist 11 and the trough 3 is doped with positive ions at a higherconcentration than in the preceding implantation. Consequently, a secondhighly positively doped surface layer 12 is obtained, as is illustratedin FIG. 5. Diffusion takes place, with the result that the positivelydoped first region 4 is obtained, as is illustrated in FIG. 6.Subsequently, the photoresist 11 is removed and the first region 4 iscovered with photoresist 11. Implantation with negative ions thenfollows, with the result that a highly negatively doped surface layer 13is produced, as is illustrated in FIG. 7. With the aid of diffusion, thehighly negatively doped second region 6 according to FIG. 1 is produced.In this way, the avalanche breakdown diode illustrated in FIG. 1 isproduced, the width of the interspace 5 between the trough 3 and thelayer 2 being precisely established.

What is claimed is:
 1. An avalanche breakdown diode having a p-typesubstrate, comprising:a first p-doped region formed in the p-typesubstrate; a second p-doped region formed in the first p-doped region,the second p-doped region having a doping concentration greater than thefirst p-doped region; a n-doped region formed in the p-type substrate; athird p-doped region formed underneath the n-doped region, the thirdp-doped region and the first p-doped region defining an interspacehaving a predetermined width and including the p-type substrate, thethird p-doped region covering at least a portion of the n-doped regionadjacent to the interspace; an insulating layer formed over theinterspace; a conducting layer applied to the insulating layer; andwherein the second p-doped region is connected to a negative voltage,the n-doped region is connected to a positive voltage and the conductivelayer is connected to the positive voltage.
 2. The avalanche breakdowndiode according to claim 1, wherein the first p-doped region is formedas a trough.
 3. The avalanche breakdown diode according to claim 1,wherein the insulating layer includes silicon oxide.
 4. The avalanchebreakdown diode according to claim 1, wherein the conductive layerincludes polysilicon.
 5. An avalanche breakdown diode having an n-typesubstrate, comprising:a first n-doped region formed in the n-typesubstrate; a second n-doped region formed in the first n-doped region,the second n-doped region having a doping concentration greater than thefirst n-doped region; a p-doped region formed in the n-type substrate; athird n-doped region formed underneath the p-doped region, the thirdn-doped region and the first n-doped region defining an interspacehaving a predetermined width and including the n-type substrate, thethird n-doped region covering at least a portion of the p-doped regionadjacent to the interspace; an insulating layer formed over theinterspace; a conducting layer applied to the insulating layer; andwherein the second n-doped region is connected to a positive voltage,the p-doped region is connected to a negative voltage and the conductivelayer is connected to the negative voltage.